Show HN: bef – a tool that encodes/decodes interleaved erasure coded streams
github.com
github.com
This is a really important property in situations where there can be big giant bursts of errors, because you can still reconstruct the data regardless. IIRC, CDs/DVDs/BDs all use two concatenated Reed Solomon (a type of erasure coding) coded symbols that are then interleaved with each other, which provides the disk protection against things like accidental scratches.
Your benchmark also doesn't list the redundancy %, as well as how resilient it is against corruption.
One thing I note is that both ISA-L and zfec use GF8, whilst PAR2 uses GF16. The latter is around twice as slow to compute, but allows for significantly more blocks/shards.
> par2cmdline[-turbo] encode: par2 c -r25 test
That command is rather unfair to PAR2 - you should add `-b48 -n2 -u` to make the comparison fairer.
PAR2 ain't exactly fast, particularly compared to GF8 focused formats, but the numbers you initially gave seemed wildly off, so I suspected the comparison wasn't really fair.
Ideally you should also be including the version of each tool used.
It seems par2 is significantly faster with those options set than without, as in by an order of magnitude, it seems par2 struggles greatly with the large number of blocks that it sets by default. Thank you for telling me.
Yeah, the compute complexity for Reed Solomon is generally O(size_of_input * number_of_recovery_blocks)
If you don't specify the number of blocks, par2cmdline defaults to 2000, so at 25%, it's generating 500 parity blocks, which is obviously much slower than what you're generating with the other tools.
Having said that, PAR2 is generally aimed at use cases with hundreds/thousands of parity blocks, so it's going to be at a disadvantage if you're generating less than 10 - which your benchmark shows.
I don’t know how that figures into your decision to name it this but at least now you’re aware.
...And then copyright them all. Muhahaha.
In this case, it’s not a great source. There are two, low-ranking Dutch definitions but they’re very direct or vulgar.
What is a better sequence of steps in your opinion?
tar c dir | zstd | gpg -e | bef -c -o backup.tar.zst.gpg.bef
and then to get back that file with the terribly long filename
bef -d -i backup.tar.zst.gpg.bef | unzstd | gpg -d | tar x
Since your head is in the thick of this problem, I’d recommend you look at seqbox and consider implementing sbx headers and blocks as an optional container that would give you resilience to filesystem corruption. That way your tool would be an all in one bitrot safeguard and streaming/pipe based!
Regarding zbackup, it’s perhaps a bit obscure but extremely useful tool for managing data. The way I use it I’m able to get both dedup and lazy incremental backups, although with a computational cost, but not so significant. The encryption is a nice side effect of its implementation that is also handy.
> bef -d -i backup.tar.zst.gpg.bef | unzstd | gpg -d | tar x
Should probably be
> bef -d -i backup.tar.zst.gpg.bef | gpg -d | unzstd | tar x
You mention how the parameters are all customizable but I want to ask almost the opposite: is there a recommend set of defaults for xxx situation that the user can apply, so they don't have to be experts to figure out usage?
e.g. a recommended option for "sharing over the internet" vs "burning to a dvd" vs "writing to tape"
(I'm aware that these have their own redundancies/error control, but obviously I do not consider them sufficient.)
bef -c --default share -i input -o output, bef -c --default dvd -i input -o output, bef -c --default tape -i input -o output, etc.
It seems like a good idea and wouldn't exactly be hard to implement.
The dependency for doing erasure codes is itself pretty interesting[1]. It has a number of backends. I've used one of those, ISA-L, in the past at a major storage vendor for Reed Solomon parity blocks.
Fountain codes have historically been patent encumbered (invented in the early 2000s) in a way that Reed Solomon (1960s) is not. So most open source software does not use them. We might see more use going forward with those patents expiring.
It wasn't immediately clear from the description -- can you tune _where_ the parity blocks are stored, to make sure that a corruption doesn't knock out the parity too? I guess this would be the interleave distance, is that the term?
In terms of controlling how many blocks and their respective fragments are interleaved, that can be controlled by the -l argument in the command. I'll paste the info I have in the manpage here.
>The number of blocks(specifically their fragments) to interleave. The default number is 3, as it provides protection for a bad burst to corrupt both the block in front of and behind it.
In general, you can approximate the burst size ratio needed to destroy a set of interleaved blocks beyond repair via this equation (this may not be fully accurate as its napkin math)
let n = number of blocks to interleave, B = number of bytes per fragment, m = number of parity fragments, k = number of required fragments to rebuild a block.
((n - 1) * m * B + n - 1) / (n * (k + m) * B)
this is because a given corruption could traverse a whole fragment and then leech into another, taking some other fragments with it with the most unlucky of bursts. When you remove the B and take the limit as n goes to infinity, it approaches the ratio of m/(k+m), or in other words as you interleave more and more blocks, you get a maximal burst size closer and closer to the size of your total number of parity fragments.
Also, the header specifies exactly the size of each fragment, we don't depend on the fragments themselves to tell us their size. In fact, the only metadata a fragment has is its hash and the number of padded bytes for the whole interleaved set of fragments, the format is (almost, ignoring padded bytes) completely described from the header as a design principle. That's why a whole fragment can be turned to garbage and we don't care, just skipping ahead to the next fragment.
edit: thinking on it, I could add a mode of behavior such that it recovers as much as possible if a specific block is corrupted, rather than the situation right now where it quits streaming after it finds a corrupted block.
> in other words as you interleave more and more blocks, you get a maximal burst size closer and closer to the size of your total number of parity fragments.
I think this could be called out more clearly, as my default assumption was this limit case -- the file grows by X amount, I can recover from up to X amount of corruption. It sounds like that's only the case if the interleave is set to maximum, which is not the default.
Is that because, as you increase the interleave, it increases the amount of the stream you need to read in order to repair any corruption?
Yes, as you need to read the entire set of interleaved blocks to get each respective block, so to keep memory consumption low, we don't want to interleave too many blocks. I could increase the default to something higher though.
Regarding the tape problem, I was being a bit daft in my response, as if you lose a chunk of tape, you also lose some indeterminate number of data with it, which my format currently isn't capable of recovering from. I'll try to fix that to some degree in the next version, at the cost of not guaranteeing backwards compatibility as I feel it falls under that major problem comment I made in the v0.1 release.
edit. ok 10 minutes later if have persuaded automake/conf/etc to create a makefile. now xxhash wont compile because src/bef.c:278:2: error: unknown type name ‘XXH128_hash_t’; did you mean ‘XXH32_hash_t’?
edit ok many more minutes later i purged ubuntu xxhash and installed my own copy and re-negotiatied with automake/conf/etc
edit lol downvoted for asking how to build.
edit ok now that its built, havent the foggiest how to use it. no example or hello world is given in readme.
edit nevermind figured it out. ./bef -c -i bef -o bef.test
edit so i still dont understand it. i bef'ed the Makefile, removed a character, tried to 'deconstruct' it, the output is zero bytes
I can't reproduce this. These are the commands I used, with it freshly compiled on a ubuntu docker container, both the v0.1 release and the master tree.
./bef -c -i Makefile -o Makefile.bef
dd if=/dev/zero of=Makefile.bef bs=1 oseek=300 count=1
./bef -d -i Makefile.bef -o Makefile2
cmp Makefile Makefile2 || echo "failed!"
edit: oh, I see, you 'removed a character'. Depending on what character you removed or corrupted from the output, you could've either hit the issue described above with inserting noise, but this time removing information, or you caused corruption in the header by either corrupting the magic number, hash type, or hash itself. The command line utility automatically truncates the output before calling the deconstruct function. The header is sadly the biggest single point of failure in the tool/format, which is why I introduced the --raw flag for those who don't want it.
dd if=/dev/urandom of=x bs=1M count=2
./bef -c -i x -o x.bef
dd if=/dev/zero of=x.bef bs=1 seek=20000 count=100 conv=notrunc
./bef -d -i x.bef -o x2
sha256sum x x2
i also tried count of 1000 and even 10000 and it still worked!!!! pretty awesome
Mind you, bursts greater than a certain size, around 8000 bytes for the defaults, are a game of probability in what damage they will do. In most cases it'll work like you did for you, but in some unlucky offsets 10000 byte corruptions with default settings could do something like this.
1 byte corrupts fragment x of block n, 4096 bytes decimate fragment x of block n+1, 4096 bytes decimate fragment x of block n+2, 1807 bytes corrupts fragment x+1 of block n.
The defaults only have 1 parity fragment, so the fact two fragments were corrupted for block n means it can't be faithfully reconstructed. In the average case you should be fine, but it is something I should probably communicate more effectively in the README (I only hint at it with the 8K per 192K comment).